There are construction machines (hydraulic excavators, wheel loaders, etc.) designed with the purposes of energy saving (low fuel consumption) and reduction in the amount of emission of exhaust gas that is discharged from the engine and leads to increase in the environmental load (carbon dioxide, nitrogen oxides, particulate matter, etc.). Such construction machines include the so-called “hybrid construction machines” which are equipped with not only the engine but also a motor generator as a source of the power.
As a technology for the hybrid construction machines, Japanese Patent No. 4633813 has disclosed a technology for avoiding the drop in the engine combustion efficiency and preventing the generation of black smoke (exhaust gas). This technology attempts to prevent the engine from sharply increasing the power by gradually increasing an upper limit value of the engine output according to a certain rate of increase, thereby reducing the amount of the exhaust gas.
A technology disclosed in Japanese Patent No. 4512283 is configured to adjust excess and deficiency of the power supplied by the engine with respect to the demanded power of the hydraulic pump through the charging/discharging operation of the motor generator. Thus this technology attempts to improve the fuel efficiency through the prevention of oversupply of the total power of the engine and the motor generator and also to prevent the engine stall attributable to undersupply of the power to the hydraulic pump.
In a technology disclosed in JP-2003-9308-A, it is possible for the operator to select a scheme of completely stopping the engine and having the construction machine driven by the motor generator alone. While such scheme is selected, the operation can be performed without consuming the fuel or discharging the exhaust gas.